1. Field of the Invention
The present invention relates to a circuit breaker, and more particularly, to a circuit breaker including a moving contact assembly which corrects the position of a moving contact depending positional errors of points of contact during the ON operation and return the moving contact to the normal position when the circuit is interrupted.
2. Description of the Conventional Art
In general, a circuit breaker is an electrical device designed to manually open and close an electric circuit using a handle or to protect a load device and a circuit by detecting a fault condition such as short circuit and automatically interrupting the circuit.
FIG. 1 is a cross-sectional view showing a conventional circuit breaker. FIG. 2A is a cross-sectional view showing the internal structure of a moving contact assembly of FIG. 1. FIGS. 2A and 2B will collectively be referred to herein as FIG. 2.
As shown in FIGS. 1 and 2, the conventional circuit breaker includes fixed contact points 10 fixedly mounted within a case C, a moving contact assembly A rotatably mounted to be brought into contact with or separated from the fixed contact points 10, and a switching mechanism 70 that generates driving force to bring the moving contact assembly A into contact with the fixed contact points 10 or separate it from the fixed contact points 10.
The fixed contact points 10 are arranged in a pair symmetrically with respect to the rotation axis of a shaft 20 to be described later.
The moving contact assembly A includes the shaft 20 that is rotatable in a first direction or a second direction opposite to the first direction by means of the switching mechanism 70, a moving contact 30 that is held to be rotatable in the first or second direction, independently from the rotation of the moving contact assembly A by the switching mechanism 70, with respect to the shaft 20, with the rotation axis not fixed to the shaft 20, and springs 50 that apply torque to the moving contact 30 in the first direction with respect to the shaft 20. The first direction is a counterclockwise direction in the drawings, in which the moving contact assembly A is brought into contact with the fixed contact points 10.
The shaft 20 includes stopping walls 24 that stop the rotation of the moving contact 30 in the first direction and guides the moving contact 30 to the normal position. The stopping walls 24 each includes a stopping face 24a that is formed in the direction opposite to the first direction in which the moving contact 30 rotates, and a guiding face 24b that is curved from the stopping face 24a, is shaped like an arc bulging toward the rotation axis of the shaft 20 when viewed from a cross-section perpendicular to the rotation axis of the shaft 20, and faces the rotation axis of the shaft 20. The stopping walls 24 are arranged in a pair symmetrically with respect to the rotation axis of the shaft 20.
The moving contact 30 includes first surfaces 34a that are formed along the radius of rotation of the moving contact 30 and come into contact with the stopping faces 24a, and sliding surfaces 32a that extend in a curve from the first surfaces 34a and bring the guiding faces 24b into internal contact with them.
The sliding surfaces 32a are curved such that the center of curvature of the sliding surfaces 32a coincides with the center of curvature of the guiding faces 24b when the moving contact 30 is held in the shaft 20.
The first surfaces 34a and the sliding surfaces 32a are arranged in pairs symmetrically with respect to the rotation axis of the moving contact 30.
With this configuration, when a handle 72 is turned in the counterclockwise direction in the drawings to the ON operation, the moving contact assembly A rotates in the counterclockwise direction in the drawings by means of the switching mechanism 70 and comes into contact with the fixed contact points 10. That is, a circuit connection is established.
On the other hand, if the user manually closes the circuit by turning the handle 72 in the clockwise direction in the drawings, or the circuit is closed when a tripping mechanism 74 of the switching mechanism 70 is actuated due to a failure such as an abnormal current in a line, the moving contact assembly A rotates in the clockwise direction in the drawings by means of the switching mechanism 70 and therefore disconnected from the fixed contact points 10. That is, the circuit is interrupted.
In this procedure, the moving contact 30 receives torque from the springs 50 when disconnected from the fixed contact points 10. Accordingly, the sliding surfaces 32a come into contact with the guiding face 24b, and a tangential force F of the torque is exerted on the sliding surfaces 32a at the points of contact. The component force (F′×cos θ′ ) directed toward the sliding surfaces 32a acts as the force for returning the moving contact 30 to the normal position. By this force, the sliding surfaces 32a move with respect to the guiding faces 24b to allow the moving contact 30 to return to the normal position. The normal position is the position at which the rotation axis of the moving contact 30 coincides with the rotation axis of the shaft 20.
By the way, in the conventional circuit breaker, the sliding surfaces 32a are curved to come into internal contact with the guiding faces 24b, and this causes the sliding surfaces 32 and the guiding faces 24b to be in contact with each other, with the line of action of the force F and the sliding surfaces 32a being perpendicular or near perpendicular to each other, while the moving contact 30 has not returned to the normal position. In this case, the component force (F×cos θ) directed toward the sliding surfaces 32a becomes zero (0) or a lower value than a frictional force, which leads to a lack of the returning force. As a result, a positional error may occur, by which the moving contact 30 cannot return to the normal position, and a contact failure may occur even if the moving contact 30 is released from the off-normal position and put into operation.